U.S. patent number 5,319,634 [Application Number 07/773,009] was granted by the patent office on 1994-06-07 for multiple access telephone extension systems and methods.
This patent grant is currently assigned to Phoenix Corporation. Invention is credited to David B. Bartholomew, A. Ray Ivie, Alma K. Schurig.
United States Patent |
5,319,634 |
Bartholomew , et
al. |
June 7, 1994 |
**Please see images for:
( Certificate of Correction ) ** |
Multiple access telephone extension systems and methods
Abstract
A method and system for conducting multiple access simultaneous
telephone communications in full duplex either over the power lines
of a building or using RF transmission. It employs a combination of
multiple access techniques selected from among the following: time
division, code division, and frequency division. The following
features result: a) security coding to prevent unauthorized access
and eavesdropping, b) multiple simultaneous conversations through
identical and closely coupled transmission media, c)
non-interference to other communications systems and users, and d)
processing gain for operating in noisy environments.
Inventors: |
Bartholomew; David B. (West
Valley City, UT), Ivie; A. Ray (Orem, UT), Schurig; Alma
K. (Provo, UT) |
Assignee: |
Phoenix Corporation (Midvale,
UT)
|
Family
ID: |
25096902 |
Appl.
No.: |
07/773,009 |
Filed: |
October 7, 1991 |
Current U.S.
Class: |
370/441; 370/436;
370/442; 455/402 |
Current CPC
Class: |
H04M
1/71 (20210101); H04J 13/00 (20130101); H04B
3/542 (20130101); H04M 1/733 (20130101); H04B
2203/5425 (20130101); H04B 2203/5483 (20130101); H04B
2203/545 (20130101); H04B 2203/5491 (20130101); H04B
2203/5437 (20130101); H04B 2203/5441 (20130101); H04B
2203/5408 (20130101); H04B 2203/5445 (20130101) |
Current International
Class: |
H04M
1/733 (20060101); H04M 1/72 (20060101); H04J
13/02 (20060101); H04B 3/54 (20060101); H04J
13/00 (20060101); H04B 007/204 (); H04J 013/00 ();
H04J 003/16 () |
Field of
Search: |
;370/11,18,24,29,30,69.1,95.3,50,70,95.1 ;375/1,38,40
;379/58,59,60,61 ;380/34
;455/33.1,33.2,34.1,54.1,49.1,53.1,56.1 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Olms; Douglas W.
Assistant Examiner: Hsu; Alpus H.
Attorney, Agent or Firm: Christiansen; Jon C. Hollaar; Lee
A. McCarthy; Daniel P.
Claims
What is claimed and desired to be secured by United States Letters
Patent is:
1. A telephone communications system, the system comprising:
(a) a base unit comprising:
(i) a first transmitter subsystem,
(ii) a first receiver subsystem, and
(iii) means for connecting said first transmitter subsystem and
said first receiver subsystem to a telephone line;
(b) an extension unit comprising:
(i) a second transmitter subsystem, and
(ii) a second receiver subsystem; each of said first and second
transmitter subsystems comprising:
(a) means or converting analog telephone communications signals to
digital data signals;
(b) means for generating a transmission carrier controlled by at
least one multiple access means, said at least one multiple access
means being selected form the group consisting of:
(i) time division multiple access multiplexing means for
controlling the times at which the digital data signals are
transmitted; and
(ii) frequency division multiple access means for changing said
transmission carrier's frequency;
(c) means for generating a pseudonoise code;
(d) means for modulating said transmission carrier by said digital
data signals and said pseudonoise code to produce a direct spread
code division multiple access signal; and
(e) means for coupling said direct spread code division multiple
access signal to a communications medium;
and each of said first and second receiver subsystems
comprising:
(a) means for receiving said direct spread code division multiple
access signal from said communications medium;
(b) means for recovering said modulated transmission carrier from
said received direct spread code division multiple access signal;
and
(c) means for demodulating said recovered modulated transmission
carrier to produce digital data corresponding to said analog
telephone communications signals.
2. A telephone communications system as in claim 1 further
comprising off hook circuitry for detecting off hook status in said
extension unit and placing said telephone line connecting means of
said base unit into an off hook condition.
3. A telephone communications system as in claim 1, said base unit
further comprising ring detect circuitry for detecting ring signals
on said telephone line and transmitting a ring command to said
extension unit; and said extension unit further comprising ring
generator circuitry for generating a ring signal in said extension
unit upon receipt of said ring command.
4. A telephone communications system as in claim 1, wherein said
means for coupling said direct spread code division multiple access
signal to a communications medium comprises an RF antenna.
5. A telephone communications system as in claim 1, wherein said
means for coupling said direct spread code division multiple access
signal to a communications medium comprises a connection to power
lines of a building.
6. A telephone communications system as in claim 1, further
comprising one or more additional extension units.
7. A telephone communications system as in claim 1, wherein said
telephone line connecting means of said base unit connects a
plurality of telephone lines to said transmitter subsystem and said
receiver subsystem.
8. A telephone communications base unit comprising:
(a) a transmitter subsystem,
(b) a receiver subsystem, and
(c) means for connecting said transmitter subsystem and said
receiver subsystem to a telephone line;
said transmitter subsystem comprising:
(a) means for converting first analog telephone communications
signals to first digital data signals;
(b) means for generating a first transmission carrier controlled by
at least one multiple access means, said at least one multiple
access means being selected from the group consisting of:
(i) time division multiple access multiplexing means for
controlling the times at which said first digital data signals are
transmitted; and
(ii) frequency division multiple access means for changing said
transmission carrier's frequency;
(c) means for generating a pseudonoise code;
(d) means for modulating said first modulated transmission carrier
by said first digital data signals and said pseudonoise code to
produce a first direct spread code division multiple access signal;
and
(e) means for coupling said first direct spread code division
multiple access signal to a communications medium;
and said receiver subsystem comprising:
(a) means for receiving a second direct spread code division
multiple access signal from said communications medium;
(b) means for recovering a second modulated transmission carrier
from said second direct spread code division multiple access
signal; and
(c) means for demodulating said second modulated transmission
carrier to produce second digital data corresponding to second
analog telephone communications signals.
9. A telephone communications base unit as in claim 8 further
comprising off hook circuitry for placing said telephone line
connecting means of said base unit into an off hook condition.
10. A telephone communications base unit as in claim 8, said base
unit further comprising ring detect circuitry for detecting ring
signals on said telephone line and transmitting a ring command to
an extension unit.
11. A telephone communications base unit as in claim 8, wherein
said means for coupling said direct spread code division multiple
access signal to a communications medium comprises an RF
antenna.
12. A telephone communications base unit as in claim 8, wherein
said means for coupling said direct spread code division multiple
access signal to a communications medium comprises a connection to
power lines of a building.
13. A telephone communications base unit as in claim 8, wherein
said telephone line connecting means of said base unit connects a
plurality of telephone lines to said transmitter subsystem and said
receiver subsystem.
14. A telephone communications extension unit comprising:
(a) a transmitter subsystem, and
(b) a receiver subsystem;
said transmitter subsystem comprising:
(a) means for converting first analog telephone communications
signals to first digital data signals;
(b) means for generating a first transmission carrier controlled by
at least one multiple access means, said at least one multiple
access means being selected form the group consisting of;
(i) time division multiple access multiplexing means for
controlling the times at which said first digital data signals are
transmitted; and
(ii) frequency division multiple access means for changing said
transmission carrier's frequency;
(c) means for generating a pseudonoise code;
(d) means for modulating said first modulated transmission carrier
by said first digital data signals and said pseudonoise code to
produce a first direct spread code division multiple access signal;
and
(e) means for coupling said first direct spread code division
multiple access signal to a communications medium;
and said receiver subsystem comprising:
(a) means for receiving a second direct spread code division
multiple access signal from said communications medium;
(b) means for recovering a second modulated transmission carrier
from said second direct spread code division multiple access
signal; and
(c) means for demodulating said second modulated transmission
carrier to produce second digital data corresponding to second
analog telephone communications signals.
15. A telephone communications extension unit as in claim 14, said
extension unit further comprising ring generator circuitry for
generating a ring signal in said extension unit upon receipt of a
ring command from a base unit.
16. A telephone communications extension unit as in claim 14,
wherein said means for coupling said direct spread code division
multiple access signal a communications medium comprises an RF
antenna.
17. A telephone communications extension unit as in claim 14,
wherein said means for coupling said direct spread code division
multiple access signal to a communications medium comprises a
connection to power lines of a building.
18. A method for telephone communications between a first unit and
a second unit, the method comprising the steps of:
(a) said first unit converting analog telephone communications
signals to digital data signals;
(b) said first unit generating a transmission carrier controlled by
at least one multiple access technique selected form the group
consisting of:
(i) time division multiple access for controlling the times at
which the transmission carrier is transmitted; and
(ii) frequency division multiple access for changing said
transmission carrier's frequency;
(c) said first unit generating pseudonoise code;
(d) said first unit modulating said transmission carrier with said
digital data signals and said pseudonoise code to produce a direct
spread code division multiple access signal;
(e) said first unit transmitting said direct spread code division
multiple access signal to said second unit;
(f) said second unit recovering said modulated transmission carrier
from said direct spread code division multiple access signal;
and
(g) said second unit demodulating said modulated transmission
carrier to produce digital data corresponding to said analog
telephone communications signals.
19. A method for telephone communications as in claim 18, further
comprising:
(h) connecting one or more telephone lines to said first unit with
corresponding ring, off hook and duplex audio signals to provide
said first unit with a base unit capability.
Description
BACKGROUND
1. The Field of the Invention
This invention relates generally to digital communication systems
and, particularly, to such systems which provide for multiple
access to a plurality of signals carried on a single communications
medium. More specifically, this invention relates to telephone
extension systems, by which signals are transferred simultaneously
between a plurality of telephone lines and telephone extensions by
means of the AC power lines of a building or an RF (radio
frequency) transmission medium.
2. The Background Art
When conventional telephone systems are installed in a building, a
significant expense is frequently associated with running the
necessary telephone wires for all desired telephone extensions. In
an existing building, the telephone installation process may also
significantly disrupt the building's normal use. Moreover, due to
the time and expense involved, the installation is very often not
susceptible to convenient modification, despite changes in the
needs of the telephone system user.
In an effort to overcome the foregoing disadvantages, various types
of wireless telephone systems have been developed. Wireless
telephone systems typically include a base unit which receives the
telephone signal from a conventional telephone line. The signal is
then transmitted between the base unit and one or more extension
locations in some manner. Most commonly, the telephone signal is
transmitted between the base unit and the extensions using
conventional radio frequency (RF) transmission signals and
techniques. More recently, however, attempts have been made to
transmit the telephone signal using the existing power lines of the
building. These prior efforts have had varying degrees of
success.
For example, one of the major deterrents in transmitting telephone
signals over existing power lines is the nature of the power line
medium itself, which presents a low and variable impedance to
carrier signals as well as an extremely noisy communications
environment. Studies have demonstrated that the optimum carrier
frequency range lies between 3 and 15 MHz. Most prior art attempts
to operate below 2 MHz have failed commercially because of noise or
interference problems from other equipment operating on the
electrical system.
Numerous prior art signal modulation techniques have also been
attempted, primarily employing FM modulation of the carrier by
audio (speech) signals (U.S. Pat. Nos. 3,949,172 and 4,701,945
being examples, the disclosures of which are incorporated herein by
this reference). The problem with FM modulation is that no security
is afforded the users; that is, other users with the same devices
can make calls on another user's line and eavesdrop on
conversations. The impact of these problems has already been
demonstrated in the cordless telephone industry, which shares the
same limitations as the line carrier industry. Additionally,
commercial AM and FM broadcast stations are often heterodyned and
demodulated in the RF range utilized by these systems, thereby
interfering with the reception of telephone conversations. The
transmissions from one of these systems will often also radiate and
interfere with other types of FCC licensed and unlicensed
commercial and residential equipment. Even the use of two FM
modulation stages, as described in U.S. Pat. No. 4,701,945, is not
able to solve these problems.
In prior art systems, full duplex voice communication is usually
attempted by using two carrier frequencies, one for each direction.
Usually a transmitter and receiver are included in each station
which are operating simultaneously. This leads to mutual
interference as well as increasing the normal interference drift
problems and does not eliminate the security problems.
Recently, attempts have been made to transmit relatively low
frequency digital data (<2 Kbs) via a line carrier and employing
a multiple access technique known as direct spread. (See, for
example, U.S. Pat. Nos. 4,641,322 and 4,864,589, the disclosures of
which are incorporated herein by this reference.) Generally, the
carrier frequencies (200-500 KHz) and corresponding data rates
(20-1000 bs) are too low to provide sufficient processing gain to
permit real time full duplex voice communication which generally
requires about 100 Kbs. The systems using direct spread techniques
also typically employ line carrier remote data collection and
control applications for which high speed multiple channel data
transmission is not required. Such systems likewise do not
typically accommodate more than one system using the same power
lines in the same building.
In summary, therefore, no prior art line carrier telephone
extension system is known which permits private, multiple line,
high quality duplex voice communications which does not interfere
with other electronics systems.
BRIEF SUMMARY AND OBJECTS OF THE INVENTION
In view of the foregoing, it is a primary object of the present
invention to provide an effective method of multiple access
communication which provides for multiple access of a plurality of
signals on a single communications medium.
It is also an object of the present invention to provide a method
and system of line carrier communications utilizing both TDMA (time
division multiple access) and CDMA (code division multiple access)
to permit high data rates and multiple access by two or more
telephone lines.
Further, it is an object of the present invention to provide a
method and system of line carrier telephone communications which
utilizes CDMA (code division multiple access) to provide a high
degree of security for preventing unauthorized access to the
subscriber's line, and which provides privacy with respect to the
conversation from third parties.
It is a still further object of the present invention to provide a
method and system of code synchronization to provide multiple
extensions for the same subscriber line which do not interfere with
each other.
An additional object of the present invention is to provide a
method and system of line carrier telephone communications which
utilizes FDMA (frequency division multiple access) in combination
with CDMA (code division multiple access) to prevent interference
between relatively close neighboring transmission systems or
partner transmissions in the same system and to provide for
multiple access (simultaneous transmission) of duplex signals for
at least one telephone line.
Also, it is an object of the present invention to provide a method
and system of multiple access cordless telephone extension
communications which applies the same techniques to obtain the same
advantages as for the line carrier telephone extension systems and
methods.
Consistent with the foregoing objects, and in accordance with the
invention as embodied and broadly described herein, a telephone
communications system and method is disclosed in one embodiment of
the present invention for conducting multiple access simultaneous
telephone communications in full duplex either over the power lines
of a building or over a common RF transmission means. The method
employs a combination of multiple access techniques selected from
among the following: time division, code division, and frequency
division. The following features result: a) security coding to
prevent unauthorized access and eavesdropping, b) multiple
simultaneous conversations through identical and closely coupled
transmission media, c) non-interference to other communications
systems and users, and d) processing gain for operating in noisy
environments. The method also relates to improvements in cordless
telephone communication.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects and features of the present
invention will become more fully apparent from the following
description and appended claims, taken in conjunction with the
accompanying drawings. Understanding that these drawings depict
only typical embodiments of the invention and are, therefore, not
to be considered limiting of its scope, the invention will be
described with additional specificity and detail through use of the
accompanying drawings in which:
FIG. 1 is a block diagram of a line carrier telephone extension
system in accordance with one presently preferred embodiment of the
present invention, the system servicing a plurality of subscriber
lines and corresponding extension phone sets;
FIG. 2 is a block diagram of a line carrier PABX telephone
extension system in accordance with one presently preferred
embodiment of the present invention, the system servicing a
plurality of line carrier extension phones and conventional
extension phones;
FIG. 3 a is multiple access signal coverage diagram illustrating
the deployment and combination of multiple access techniques for
solving near-far problems and simultaneous use of a transmission
medium in accordance with one presently preferred embodiment of the
present invention;
FIG. 4 is an electrical block diagram of a base unit of a multiple
access line carrier telephone extension system in accordance with
one presently preferred embodiment of the present invention, said
base unit interfacing a plurality of subscriber lines to the power
line distribution system of a building;
FIG. 5 is an electrical block diagram of an extension unit of a
multiple access line carrier telephone extension system in
accordance with one presently preferred embodiment of the present
invention, the extension unit interfacing an extension phone to the
power line distribution system of a building; and
FIG. 6 is a schematic and block diagram illustrating one presently
preferred embodiment of the base unit system diagram of FIG. 4,
including the associated transmitter and receiver subsystems.
FIG. 7 (i.e. FIGS. 7A, 7B, 7C, 7D and 7E) is a schematic diagram of
base unit subsystems described in FIG. 6.
FIG. 8 is a block diagram of an extension unit system controller
and digital data multiplexer, including the associated transmitter
and receiver subsystems.
FIG. 9A and 9B are a complete schematic diagram of the PN generator
employed in FIGS. 6 through 8.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
It will be readily understood that the components of the present
invention, as generally described and illustrated in the Figures
herein, could be arranged and designed in a wide variety of
different configurations. Thus, the following more detailed
description of the embodiments of the system and method of the
present invention, as represented in FIGS. 1 through 9, is not
intended to limit the scope of the invention, as claimed, but it is
merely representative of the presently preferred embodiments of the
invention.
The presently preferred embodiments of the invention will be best
understood by reference to the drawings, wherein like parts are
designated by like numerals throughout.
It will be readily apparent from the discussion which follows that
the present invention is adapted for use in a wide variety of
specific communications applications, including telephone
communications. The present invention may likewise be used with
virtually any communications medium, such as, for example, RF
signals or line carriers such as the power lines in a building.
Nevertheless, in order to simplify the following discussion, the
presently preferred embodiments of the present invention will be
described below with specific reference to a telephone
communications system which employs line carriers as the
communications medium.
An increasing number of people live in apartments and condominiums
which share power lines. It is, therefore, important for a
telephone extension system to utilize multiple access techniques in
a unique and skillful manner so as to permit acceptable operation.
Many prior art limitations can be overcome by proper application of
multiple access techniques as employed in the instant
invention.
The following specific multiple access (MA) techniques are employed
in the presently preferred embodiments of the present
invention:
A) Time Division Multiple Access (TDMA), and two spread spectrum
techniques, namely:
B1) Code Division Multiple Access (CDMA), often called direct
spread, which can include subcarrier CDMA, and
B2) Frequency Division Multiple Access (FDMA), which includes
frequency hopping techniques and deployment of multiple
simultaneous transmission frequencies.
These techniques are more fully described in a work by Robert C.
Dixon, "Spread Spectrum Systems," 2nd Ed., John Wiley & Sons,
(U.S.A., 1984), which is made a part hereof by reference.
TDMA (time division multiple access) is employed in the instant
invention to permit the bidirectional transmission of duplex voice
data for one or more subscriber lines which are "partners" in the
same multiline installation. A single base unit interfaces with the
subscriber lines and transmission medium (AC power line) and
controls the transmission of bidirection 1 voice data by breaking
the transmissions into time frames and windows, in which each
communications channel is assigned a specific transmit and receive
time window. By this means the system controller can guarantee that
only one transmission will occupy the medium at one time. The
frequency with which each frame of windows is repeated (20-40 KHz)
is high enough to transfer high speed voice data.
CDMA (code division multiple access) provides four benefits:
security, privacy, low interference with other FCC licensed systems
and antijamming margin from interference and competing users. The
fact that neighboring users may wish to make simultaneous use of
the transmission medium requires that the transmission be coded so
that a casual neighbor cannot access (transmit to) another's phone
line to make long distant calls ("security") or intercept
(eavesdrop/receive) their conversation ("privacy"). CDMA also
spreads the radiation spectrum of the transmission energy over a
very wide bandwidth (2-10 MHz) so that the energy content of any
licensed or unlicensed narrow band is too small to interfere with
FCC licensed users. CDMA also rejects many types of power line
interference due to inherent "processing gain"; and the multiple
access feature of using mutually orthogonal codes prevents jamming
interference between users of similar telephone extension systems
if their signals are below the "jamming margin". A discussion of
these terms follows.
Processing gain (Pg) is acquired by using more bandwidth than the
data requires. Processing gain is defined by the following
equation: ##EQU1##
Thus, if the RF bandwidth is 5 MHz and the data bandwidth is 100
KHz then the processing gain would be 50 times or 17 dB. Since
approximately 6 dB of gain is required for correlation and
demodulation, the resultant 11 dB is the jamming margin (Mj) (i.e.,
Mj=Pg-6 dB). A 10 dB Mj would provide that ten jamming sources of
equal strength or distance to the "friendly" signals could be
rejected, or one "unfriendly" source of equal strength ten times
closer, or one unfriendly source ten times greater in strength and
equal in distance. For this reason, CDMA cannot be relied upon to
carry the burden of jamming margin for near jamming sources. The
CDMA jamming margin is also related to code length which is the
number of code generator clock cycles ("chips") a code generator
will cycle through before the code pattern repeats; but as long as
the code length exceeds the processing gain, the main advantage
gained by using long codes is privacy and security related.
Because of the limitations of CDMA (code division multiple access)
to provide a high antijamming margin for near neighbors, FDMA
(frequency division multiple access) is employed to the extent that
it is limited by the bandwidth of the media. Thus, if the center
frequency of near neighbors were offset by 1 to 3 MHz, the
correlation and recovery of the required IF (intermediate
frequencies) is diminished substantially.
Prior art line carrier and cordless telephone extension systems do
not combine the advantages of the forms of multiple access
technology described above. Consequently, they suffer from
significant operational disadvantages which are believed to be
overcome by system and method of the present invention, a presently
preferred embodiment of which will now be described in greater
detail.
Reference is first made to FIG. 1 which illustrates one presently
preferred embodiment of a line carrier telephone extension system
in accordance with the present invention. As shown, the line
carrier telephone extension system in FIG. 1 services a plurality
of subscriber lines 1-2 and corresponding extension phone sets 7,
10 and 13. Subscriber lines 1 and 2 plug into base unit 3 which
provides the interface circuitry and protocols for the subscriber
line signals (including ring, off hook and duplex audio signals),
and for multiple access power line carrier signals, which are
carried through a building via AC power lines 4 to extension
interface units 5, 8 and 11. The extension units 5 and 8 are single
line extensions and provide means to interface the multiple access
line carrier signals to single line phone sets 7 and 10 via lines 6
and 9, respectively. Extension unit 11 provides means to interface
the multiple access line carrier signals to a multiline phone set
13 via lines 12.
A line carrier system as illustrated in FIG. 1 must operate in a
manner which is transparent to the extension phone set user. Thus,
by means of base unit 3, AC power lines 4 and extension unit 5, an
incoming ring signal is detected in the base unit, encoded into
multiple access line carrier signals, transferred to the power
lines 4, decoded in the extension unit 5 and converted to a ring
signal for extension phone set 7.
When a user picks up the receiver of an extension phone set, it
goes off hook, which condition is detected in the extension unit 5,
encoded by said extension unit into multiple access line carrier
signals and transmitted via power lines 4 to the base unit 3 where
the line carrier signals are decoded and the subscriber line is
captured by off hook circuitry. Duplex audio circuitry then
connects to the line and bidirectional transmission of voice and/or
dial tones and DTMF (dual tone multifrequency) signals takes place,
with the voice being encoded into multiple access line carrier
signals, transmitted via power lines 4 and decoded back into voice
in the base and extension units 3 and 5, respectively. Specific
subsystems and corresponding functions will be addressed with
reference to FIGS. 4 through 8.
While in the preferred embodiment the voice signals are encoded
into multiple access line carrier signals by converting the analog
voice signals to digital representations using a analog-to-digital
converters or CODECs, an alternative embodiment of the invention
could use a frequency-modulation encoding or modulation technique.
Such encoding techniques are well-known in the art.
Alternatively, the preferred embodiment can directly transmit
digital data by eliminating the analog-to-digital converters and
directly connecting a digital input in place of the output of the
analog-to-digital converter. The preferred embodiment can also
directly produce digital output by eliminating the
digital-to-analog converter in the receiving subsystem and using
the digital signal previously connected to the digital-to-analog
converter as the digital output.
Another embodiment of a multiple access line carrier system in
accordance with the present invention is illustrated in FIG. 2 and
involves replacing conventional PAB extension lines 22 with
multiple access line carrier systems, which can be easily installed
and moved. As shown, subscriber lines 1-2 connect to PABX unit 14
which provides a plurality of extension phone lines 22 and 15.
Extension lines 15 connect to multiple access base units 3 while
some of the extension lines 22 connect in a conventional manner to
other phone sets. Bas units 3 interface the PABX signals to the AC
power lines 4 of a building, as previously described. Multiple
access extension units 5, 8 and 16 interface the line carrier
signals to phone sets 7, 10 and 18 via voice and data lines 6, 9,
17, and 19-21, said data lines 19-21 providing PABX control signals
to phone sets 7, 10, and 18, or, alternatively, digital data for
digital communications equipment, such as, for example, computers
and fax machines.
The relative advantages of using various multiple access techniques
are depicted in FIG. 3. Four apartments 71-74 are shown which are
located in the same building and which share a common power line
transformer secondary. Apartment #1 71 has two phone lines 75 which
are connected to a base unit 3 (FIG. 4) of the instant invention,
not shown. Apartment #2 72 adjoins apartment #1 71 and has one line
76 connected to a base unit 3. Apartment #3 73 separates apartment
#4 74 from apartment #2 72 and, while apartment #3 73 may have a
phone line, it is not connected to a base unit. Apartment #4 74 has
a phone line 77 connected to a base unit.
Using TDMA, different signals are sent at different times as
orchestrated by the system controller of a base unit. TDMA
techniques are, therefore, used to separate the signals from the
two "partner" lines 75 of apartment #1 so that there is no mutual
interference between them.
The other two apartments 72 and 74 have independent base units
which use the same transmission medium as apartment 71, that is,
the power lines of the same apartment building. FDMA techniques are
used to separate apartment #2 72 from the other two competitors (71
and 74), since the IF (intermediate frequency) filters of the units
are sufficient to block strong local competitors of different
frequencies, as will be described in further detail below. There is
not sufficient bandwidth in the medium to provide more than a few
alternate frequencies. Therefore, FDMA is used to avoid
interference from competitors which are relatively "near" and,
fortunately, relatively few.
A greater number of neighbors ("competitors") exists which are
relatively "far" from each other, of which apartment #4 74 and
apartment #1 71 are examples. CDMA techniques possess sufficient
jamming margin to reject interference from weaker, more "far" away,
competitors. It is also a major characteristic of CDMA to provide a
low probability of interception from eavesdropping receivers, even
at "near" distances, which is why CDMA is employed in all
cases.
A multiple access base unit 3 is functionally diagramed in FIG. 4
which illustrates subsystems selected from and providing a
combination of two or more multiple access techniques: CDMA, FDMA
and TDMA. The subscriber lines 1 and 2 each connect to respective
subscriber line interfaces 30 and 40, which each contain ring
detect circuit 31, off hook circuit 32, and duplex audio circuit
33. The ring detect circuit 31 converts an incoming subscriber line
ring signal to a control signal for the system controller 42. Off
hook circuit 32 accepts an off hook command from system controller
42 and captures the subscriber line 1. Duplex audio circuit 33
connects bidirectional audio signals between line 1 and digital
signal processing (DSP) blocks 34 and 35 which interface with
digital data MUX (multiplexer) 43 and system controller 42. The DSP
blocks include analog-to-digital and digital-to-analog converters
called codecs 34 and digital data compression/expansion circuits
35.
Alternatively, the preferred embodiment can directly transmit
digital data by eliminating the analog-to-digital converters 34 and
directly connecting a digital input in place of the output of the
analog-to-digital converter 34. The preferred embodiment can also
directly produce digital output by eliminating the
digital-to-analog converter 34 in the receiving subsystem and using
the digital signal previously connected to the digital-to-analog
converter 34 as the digital output. This could be used to provide a
direct interface to an ISDN data or facsimile device.
The TDMA (time division multiple access) subsystem 41, between the
system controller 42 and data MUX 43, multiplexes the transmit and
receive functions performed in the CDMA/FDMA (code and frequency
division multiple access) circuits with the incoming and outgoing
voice data streams and control signals for ring and off hook/busy.
For example, in one embodiment which relies primarily on time and
code division multiple access techniques, the system controller 42
divides the total system time into blocks of 10 microsecond
duration. During the first half of the block the base unit 3
transmitter is enabled, and transmits data from subscriber lines 1
and 2 in two 2.5 microsecond windows to the extension units 5, 8
and 11 (see FIG. 1). During the second half of the block, the base
unit 3 receiver is enabled and data is received from two extension
units in two 2.5 microsecond windows and transferred t subscriber
lines 1 and 2 via subscriber line interfaces 30 and 40. Short time
spaces between transmit and receive windows and modes allows for
signal propagation delays between the base and extension units.
In addition to TDMA (time division multiple access), FIG. 4
illustrates the use of CDMA and FDMA in the line carrier
transceiver section. A pseudonoise (PN) code generator 44 and
carrier frequency oscillator 56, by means of a modulator 48,
produces a spread spectrum line carrier. In one embodiment the
oscillator 56 runs at 10 MHz and the PN code rate is 5 MHz,
producing a spread spectrum main lobe bandwidth from 5 to 15 MHz.
Use of 4095+ bit (chip) code length ensures reasonable privacy and
security and minimizes interference with other types of equipment.
An FSK (frequency shift key) modulator 49 modulates the carrier
with the data to be transmitted. Some other forms of data
modulation can also work with spread spectrum modulation, such as
BPSK (biphase shift key), QPSK (quadriphase shift key), MSK
(minimum shift key) and FM (frequency modulation). Code selection
switches and oscillator/carrier frequency switches can provide user
changeability of CDMA and FDMA parameters to minimize collision
potentials between physically proximate systems.
To accommodate the variation in distances between the base unit and
the several extension units and to still maintain carrier lock with
the system oscillator, the base receiver comprises a plurality of
PN (pseudonoise) generators 45 an 46 which are delayed in time from
the transmitter PN generator 44 as selected by the system
controller via PN MUX (multiplexer) 47 to provide the correct PN
phase to the CDMA correlator 52 permitting correlation of extension
CDMA signals coming from the AC lines 4 through coupler 55 and
filter 51 to said correlator/demodulator 52. The correlated
receiver signals are amplified and filtered by IF 53 prior to
demodulation by data demodulator 54. The digital data MUX 43
insures that the right data is transferred to the right subscriber
line interface during its assigned TDMA window.
The use of TDMA (time division multiple access for transmit and
receive modes as well as for the duplex data for each subscriber
line allows the transmitters and receivers of both the base 3 and
extension units 5, 8, and 11 (see FIG. 1) to operate at the same
carrier and IF frequencies. Frequency division multiple access is
not required if no near neighbors are using the transmission
medium.
Another embodiment of the present invention does not require the
use of TDMA (time division multiple access) if FDMA (frequency
division multiple access) is employed for each data channel. The
transmitters in the base 3 and extension units 5, 8, and 11 send
data on two or more carrier frequencies simultaneously, and the
receivers have corresponding correlators 52 for each transmitted
data channel. Carrier frequencies and corresponding heterodyne
correlator frequencies can be synthesized so that single or
multiple IF's (intermediate frequencies) can be employed to reduce
mutual interference between subsystems in a unit. Differing CDMA
codes can also be used in each data channel to reduce mutual
interference with other data channels. Care must also be taken to
avoid collisions between carrier and code rate harmonics.
FIG. 5 illustrates one presently preferred embodiment of a multiple
access extension unit in accordance with the present invention. As
shown, the extension unit diagramed in FIG. 5 contains most of the
same subsystems as the base unit of FIG. 4, and the same reference
numerals are accordingly employed. There is, however, one notable
exception: the additional PN (pseudonoise) generators 45 and 46 of
the base unit are omitted in the extension unit. The same type of
PN code generator 44 which is used to correlate the received spread
spectrum carrier signal in the base unit is used in the extension
unit to CDMA modulate the transmitter data returning to the base
unit. Thus, referring to FIG. 1 and 5, multiple access carrier
signals from the base unit 3 are carried by power lines 4 to
extension units 5, 8 and 11 and coupled via AC line coupler 55 and
filter 51 into the CDMA correlator 52. The recovered data modulated
carrier is amplified and filtered by IF (intermediate frequency)
amplifier 53 and sent to data demodulator 54 where the data is
recovered and multiplexed 43 to the extension phone interface 60
where digital decompression (expansion) 65 and digital to analog
converter 64 provide duplex audio to the extension phone line 6 and
set 7. Similarly, audio returning from the extension line 7 to the
subscriber line 1 passes through duplex conversion 63, A/D
conversion 64 and DSP compression 65 to TDMA controller 57 where
data MUX 43 sends the signal in its appropriate time window to data
modulator 49, CDMA modulator 48, power amp 50 and AC line coupler
55 into power lines 4 and on its way to the base unit 3.
The system controller 58 of the extension unit of FIG. 5 can differ
from the system controller 42 of the base unit 3 in that the former
contains multiple extension arbitration logic. For example, in one
embodiment, only one extension phone set is permitted to use one
subscriber line at a time. If several extension sets are assigned
to one subscriber line and a first extension is using that line,
then when a second extension set is taken off hook, the second
extension unit will check the base unit transmissions to determine
that the assigned subscriber channel is already in use by an other
extension unit and will emit a busy tone to the second extension
phone set. Desired protocols may be installed in the system control
logic to handle various requirements. A programable logic device
can be used to implement this function and provides for
reprogramming the desired protocols.
Reference is next made to FIG. 6 and 7, which illustrate in more
detail a block diagram and schematic diagram of one preferred
embodiment derived from the functional block diagram of FIG. 4.
Those of ordinary skill in the art will, of course, appreciate that
various modifications to the block diagram of FIG. 6 and
corresponding schematic diagram of FIG. 7 may easily be made
without departing from the essential characteristics of the
invention, as described in connection with the block diagram of
FIG. 4 above. Thus, the following description of the detailed
diagrams of FIGS. 6 and 7 is intended only as an example, and it
simply illustrates one presently preferred embodiment of a
schematic diagram that is consistent with the foregoing description
of FIG. 4 and the invention as claimed herein. Components of FIGS.
6 and 7 which correspond to functional components of FIG. 4 are
designated by the same initial reference numbers. Thus, for
example, components which correspond to CDMA PN code generator
transmitter 44 of FIG. 4 are designated in FIG. 6 as 44-1, 44-2 and
44-3.
FIGS. 6 and 7 illustrate a specific embodiment of the system
controller 41 for a base unit 3 in which three multiple access
techniques are employed to provide a two line extension system,
which requires four data channels, two for each line. FDMA
(frequency division multiple access) is employed to multiple access
the transmit and receive function, wherein the transmitter 48-50
operates at 10.7 MHz and the receiver 51-53 at 6 MHz.
CDMA (code division multiple access) is employed in the transmitter
and receiver PN (pseudonoise) code generators 44-3, 45-4 and 46-4
to reduce mutual interference between the stronger local
transmitter signals and the weaker extension unit signals. In the
instant embodiment, this is accomplished by offsetting the phase
relationships of the similar code sequences of the three PN
generators relative to one another. This code phase offset is
partly accomplished as a natural consequence of the differing
distances of the extension units 5, 8 and 11 from the base unit 3
(see FIG. 1).
To obtain the four data channels, two data channels each for the
transmitter and receiver, TDMA (time division multiple access) is
employed to multiplex the data for lines 1 and 2 to the transmitter
(through multiplexer 43-1) and from the receiver (through
demultiplexer 43-2,3,4) at a 50% duty cycle as synchronized by the
PN (pseudonoise) codes and divider counter 42-1. Thus, at the
beginning of each 8191 chip code sequence, the divide by 64 line
select counter 42-1 is reset, and the 4.28 MHz chip clock from 44-2
clocks both the PN generator 44-3 and the TDMA line selector 42-1,
which alternately selects data at the rate of 67 KHz, providing a
bit rate for each channel of 33.5 KHz or 67 KBs.
A detailed description of the system controller circuitry of FIG. 6
follows. Power line interface or AC line coupler 55 connects
bidirectional carrier signals from AC power lines 4 to the
transmitter 50 and receiver 51. The output of transmitter
oscillator 56 is doubled by a conventional pulse edge detector
frequency doubler 44-1 and subsequently divided by five with
counter 44-2, producing a 4.28 MHz chip clock for clocking PN
generator 44-3, said PN generator having 13 stages and producing an
8191 chip long code sequence and a sync out pulse at the start of
each new sequence for resetting line select counter 42-1. The 4.28
MHz chip clock also clocks the line select counter 42-1, which
clocks line 1 and line 2 data multiplexers 43-1,2,3,4 as well as
seek multiplexer 43-1 and PN multiplexer 47.
Thus, line 1 and line 2 transmit data from the subscriber line
interfaces enter transmit data MUX 43-1, where the data modulates
the 10.7 MHz carrier at 49 and which is further code division
multiple access modulated at 48 and via transmitter output 50 sent
to the power line coupler 55. The receiver data for sending back to
the subscriber line interface is received from the power line 4 via
interface 55 and filter 51 and presented to CDMA correlator 52
which recovers the data modulated 6 MHz carrier 53. Separate PN
generators 45 and 46 are provided because the distance from the
base unit and resulting propagation delay for each extension unit
may differ, which causes the PN phase relationships between the
transmitter PN[0] generator 44-3 and the receivers' PN[1] 45-4 and
PN[2] 46-4 to be delayed in time.
During the initiation of communication, synchronization of each
receiver PN generator with that of the extension unit transmitter
is required. This is achieved by providing a PN code seek and lock
circuit consisting of 90 degree lag circuit 43-6 in combination
with phase multiplexers 45-1 and 46-1, seek multiplexer 43-5 and
the correlation output of data demodulator 54.
The seek function is provided at all times when correlation is not
detected, and is effected by individually retarding the phase of
the clock signal (10.7 MHz) input to each uncorrelated receiver PN
generator (PN[1] 45-2,3,4 and PN[2] 46-2,3,4) at the line select
rate by means of 90 degree lag circuit 43-6 and phase multiplexers
1 and 2 45-1 and 46-1. The 90 degree lag circuit 43-6 supplies all
four phases of the clock to the phase multiplexers, which select in
a retarding order one of the phases by means of a two bit binary
counter which is clocked by each line select pulse.
The seek mode of operation continuously and progressively retards
each receiver PN code phase by 90 degrees every 64 chips until the
data demodulator detects correlation of an incoming signal from one
of the extension units assigned to a specific line, whereupon the
seek mode for that line is converted to the operating mode for said
line and the phase multiplexer and PN generator hold the
correlating phase relationship and the transmit and receive data
channels for said line are activated. The seek multiplexer 43-5
insures that the uncorrelated channel for the unused line continues
in the seek mode until correlation for said unused line is
detected.
Many of the subsystems illustrated in FIGS. 4-6 can be implemented
with readily available and second sourced commercial components.
For example, ring detect 31, off hook circuit 32 and duplex audio
circuit 33 are elements of a subscriber loop interface circuit
("SLCI"). The codec 34 and compression function 35 can be
constructed with industry standard 2913-2917 devices. The codec can
also be a newer oversampled sigma-delta coder with DSP decimation
and companding, as described in several articles such as that by
Freedman et al., "IEEE Journal of Solid-State Circuits", Vol. 24,
No. 2 (U.S.A., April 1989), pp. 274-280, and manufactured by
AT&T as a T7510. The PN (pseudo noise) code generator can be a
13 stage modular shift register generator (MSRG) or a Gold code
sequence generator as described by Dixon at pp. 65-81 and
implemented with MC8504's (or equivalent shift register chips) and
exclusive-or gates. The CDMA modulator 48 can be as simple as an
exclusive-or gate with the carrier frequency and the PN code as its
inputs or a balanced modulator such as a Signetics NE602 or older
circuits described by Dixon at pp. 109-113. Similarly, the CDMA
correlator/demodulator 52 can be an NE602. The IF (intermediate
frequency) amplifier and filter 53 and demodulator 54 can consist
of an NE604 or a CA3089. The digital data MUX 43 consists of
digital gates or bilateral switches (CD4066). The application notes
of numerous manufacturers provide detailed examples and engineering
data on the implementation of the functions described in this
paragraph.
The TDMA (time division multiple access) system controller 42, on
the other hand, is a unique sequential and state logic machine, an
embodiment of which is described in detail above in connection with
FIG. 6. In addition, it should be appreciated that cost and size
limitations apply to discrete implementation of complex circuit
embodiments, which motivates the integration of most functions of
the present invention into a single low-cost application specific
integrated circuit (ASIC).
Following is a table which further lists specific electrical
component parts that might be used to implement the functions and
subsystems of the present invention, as illustrated and described
above in connection with FIGS. 4-6. Those skilled in the art will
readily appreciate, however, that other specific circuitry and
components may be equally well adapted to the implementation of the
principal functions of this invention. Thus, the following list of
specific components is intended only as an illustration, and merely
represents one presently preferred embodiment of the invention. For
simplicity, components are identified in the following table by the
corresponding reference numbers used in Figures and FIG. 4-9.
TABLE I ______________________________________ Specific Electrical
Components Comprising the Embodiment of FIGS. 4-6 Reference No.
Part No. Manufacturer ______________________________________ 31
MC33120 Motorola 32 MC33120 Motorola 33 MC33120 Motorola 34 2913
{Texas Instruments 35 2917 {Intel Lattice 42 42-1 74393
National,Motorola,Etc. 43 43-1 74157 43-2 7404 43-3 7474 43-4 7474
43-5 75157 43-6 7474 44 44-1 7408,-14,-32 44-2 7490 44-3 74194,
7474, 7486 45 45-1 74157 45-2 7414, 7408 45-3 7490 45-4 74194,
7474, 7486 46 46-1 74157 46-2 7414, 7408 46-3 7490 46-4 74194,
7474, 7486 47 74157 48 NE602, 7486 Signetics 49 7486 50 7404 51 RC,
LC Low Pass Filter 52 NE602 Signetics 53 NE604 Signetics 54 NE604
Signetics 54-1 55 56 57 58 61 62 63 64 65
______________________________________
An embodiment of the extension controller 57 is illustrated in FIG.
8. Power line interface 55 connects the power line 4 with the
transmitter 50 and receiver 51. The transmitter operates at a
carrier frequency of 6 MHz which is substantially different from
that of the receiver's 10.7 MHz 56-1. A sufficient difference in
carrier frequency allows for simultaneous operation of both the
receiver 51 and transmitter 50 with support from other subsystems
including filters and hybrid circuitry in power line interface 55
and a phase offset in receive and transmit pseudonoise (PN) codes
at PN code generator 44-3, said code offset providing a spreading
instead of a correlation of the transmitter signal 50 which may
bleed over into the receiver 51. While the offset PN output is
easily obtained by using the existing output of one of the last
stages of the modular shift register generator employed in the PN
generator 44-3, a separate PN generator and code could also be
employed. The use of a heterodyne correlator in receiver 52 in
combination with a frequency synsthesized local oscillator mixed
with PN code 44-3 enhances the flexibility for performing frequency
division multiple access. Frequencies of all subsystems are
carefully selected to avoid fundamental, harmonic and image
frequency interference.
In the extension unit (5) embodiment of FIG. 8 the base carrier
signal enters the filter 51 and is correlated and mixed with the PN
code at correlator 52 recovering the data modulated intermediate
frequency signal at 53, which signal is demodulated at 54 and the
data sent to data demultiplexer 43-3 and 43-4, which demultiplexer
sends the correct data to line 1 and line 2 extension phone
interfaces. Multiple extension arbitration logic 58-1 continuously
monitors the data and operating state of each line to provide a
busy tone indicator to a user of one extension unit attempting to
use a line which is being used by another extension unit. This is
possible because all extension units 5 receive data from the base
unit 3 but do not transmit data back to said base unit unless a
user takes the particular extension unit off hook and the line is
not already in use by another extension unit. Physically, the
arbitration logic controller is composed of several gates and
flip-flops. PN code correlation is provided by a seek circuit
composed of 10.7 MHz oscillator 56-1, 90 degree lag 56-2 and phase
multiplexer 43-6, which multiplexer clocks the PN generator
44-1,2,3 and periodically selects the next 90 degree retarded phase
of the oscillator during correlation seeking state of the receiver,
until the data demodulator detects a strong and correlated
intermediate frequency signal at receiver IF 53, whereupon seeking
is terminated and the base and extension PN codes are synchronized.
The sync output of the PN generator 44-3 resets the time division
multiple access line select counter 42-1, which divides the chip
(PN clock) rate by 64 or another suitable number depending on clock
rates and data rates. The line select output of counter 42-1 drives
the received data demultiplexer 43-3 as well as the transmit data
multiplexer 43-1, which selects which data modulates the 6 MHz
carrier oscillator 54-1 at transmitter 49 and is mixed with PN code
at mixer 48 for transmission back to the base unit by output
50.
Many variations on the system illustrated the figures will be
readily apparent to one of ordinary skill in the art from previous
discussions herein. One such variation is the application of the
herein described CDMA, TDMA and FDMA techniques to a cordless
telephone system, wherein an RF carrier replaces the power line
carrier and an antenna replaces the power line interface. Hence,
the communications medium is the only real difference, while the
multiple access requirements remain the same, and FIG. 3 applies to
a cordless telephone system as well as to a line carrier
system.
From the above discussion, it will be appreciated that the present
invention provides an effective method of multiple access
communication which provides for multiple access of a plurality of
signals on a single communications medium. The system and method of
the present invention also utilizes both TDMA (time division
multiple access) and CDMA (code division multiple access) to permit
high data rates and multiple access by two or more telephone lines.
CDMA (code division multiple access) is also utilized to provide a
high degree of security for preventing unauthorized access to the
subscriber's line, and which provides privacy with respect to the
conversation from third parties.
Additionally, the present invention provides a method and system of
code synchronization to provide multiple extensions for the same
subscriber line which do not interfere with each other. The system
and method of the present invention utilizes FDMA (frequency
division multiple access) in combination with CDMA (code division
multiple access) to prevent interference between relatively close
neighboring transmission systems or partner transmissions in the
same system and to provide for multiple access (simultaneous
transmission) of duplex signals for at least one telephone line.
Moreover, the present invention provides a method and system of
multiple access telephone extension communications which applies
equally well to both cordless and line carrier telephone extension
systems and methods.
The present invention may be embodied in other specific form
without departing from its spirit or essential characteristics. The
described embodiments are to be considered in all respects only as
illustrative, and not restrictive. The scope of the invention is,
therefore, indicated by the appended claims, rather than by the
foregoing description. All changes which come within the meaning
and range of equivalency of the claims are to be embraced within
their scope.
* * * * *